Recently, the demand for thinner and lighter electronic instruments has become increasingly greater, and thinning and densification of semiconductor packages and printed wiring boards has been promoted. For stably packaging electronic parts with satisfying the demand for thinning and densification thereof, it is important to prevent the warping to occur in packaging.
In packaging, one reason for the warping to occur in semiconductor packages is the difference in the thermal expansion coefficient between the laminate plate used in a semiconductor package and the silicon chips to be mounted on the surface of the laminate plate. Accordingly, for the laminate plate for semiconductor packages, efforts are made to make the thermal expansion coefficient of the laminate plate nearer to the thermal expansion coefficient of the silicon chips to be mounted thereon, or that is, to lower the thermal expansion coefficient of the laminate plate. Another reason is that the elastic modulus of the laminate plate is low, for which, therefore, it may be effective to increase the elastic modulus of the laminate plate. To that effect, for reducing the warping of a laminate plate, it is effective to lower the expansion coefficient of the laminate plate and to increase the elastic modulus thereof.
Various methods may be taken into consideration for lowering the thermal expansion coefficient of a laminate plate and for increasing the elastic modulus thereof; and among them there is known a method of lowering the thermal expansion coefficient of the resin for laminate plates and increasing the fill ration with an inorganic filler to be in the resin. In particular, high-rate filling with an inorganic filler is a method by which reduction in the thermal expansion coefficient and also enhancement of heat resistance and flame retardance could be expected (Patent Reference 1). However, it is known that increasing the inorganic filler content results in insulation reliability degradation, adhesiveness failure between resin and the wiring layer to be formed on the surface thereof, and pressing failure in laminate plate production, and increasing the filler content is therefore limited.
Some approaches have been tried to attain the intended purpose of thermal expansion coefficient reduction through selection or modification of resin. For example, a method of increasing the crosslinking density of the resin for wiring boards to thereby increase Tg thereof and to reduce the thermal expansion coefficient thereof is generally employed in the art (Patent References 2 and 3). However, increasing the crosslinking density is to shorten the molecular chain between functional groups, but shortening the molecular chain to a level overstepping a certain threshold is limitative in view of the reactivity of the resin, and may often bring about a problem in that the resin strength would be lowered. Consequently, there is also a limit on lowering the thermal expansion coefficient according to the method of increasing the crosslinking density.
As in the above, for conventional laminate plates, lowering the thermal expansion coefficient thereof and increasing the elastic modulus thereof have heretofore been tried by increasing the fill ration of the inorganic filler therein and by employing a resin having a low thermal expansion coefficient; however, these are being pushed to the limit.
As a method differing from the above, there has been made a trial of using a glass film as a layer having a thermal expansion coefficient almost the same as the thermal expansion coefficient of electronic parts (silicon chips) and laminating a resin on the glass film by pressing to thereby reduce the thermal shock stress of the resulting laminate (Patent Reference 4); however, the elastic modulus of the resin layer is low and the thermal expansion coefficient thereof is high, and therefore the method is insufficient for realizing the reduction in the warp of substrate.